Method and apparatus for the non-destructive measurement of the compressive strength of a solid

ABSTRACT

An apparatus and a method are described that determine the compressive strength of a solid by rebound. The method and apparatus have a reduced sensitivity to error sources, i.e. gravity, internal friction and operator interference (unsteady holding of the instrument). The improvements are achieved by contactless measuring of the quotient of rebound to inbound velocity recorded immediately before and after the impact. Matching the mass of the mallet ( 3 ) to the plunger ( 5 ) yields higher efficiency, less angular variation of the impact energy and lighter weight. Added benefits of the invention are: Extended range of measurement and simplified mechanical adjustments, calibration and maintenance.

TECHNICAL FIELD

The invention relates to a method and apparatus for the non-destructivemeasurement of the compressive strength of a solid, in particular ofconcrete.

BACKGROUND ART

Since the fifties methods have been in use to assess the compressivestrength of concrete using an apparatus with a defined tip to impactupon the surface to be tested.

The best known method is the so-called Schmidt hammer that generates adefined impact energy by extending a spring and letting it drive amallet. This mallet in turn hits upon a plunger that transfers theimpact to the surface to be tested.

Upon impact the concrete is compressed and part of the energy isabsorbed by plastic deformation. The remaining energy is returned andcauses the plunger to rebound. The rebound is then transferred back tothe mallet. The mallet then compresses the spring until the kineticenergy of the mallet is fully transferred into deformation energy of thespring. The point of this maximum compression of the spring isregistered by means of a drag pointer. Its position is readable from theoutside of the instrument. The reading of this instrument is expressedas R-value, meaning the maximum rebound travel of the mallet. TypicallyR-values range from R=20 to R=55.

FIG. 3 shows how the rebound values of the mallet can be converted to anindication of compressive strength. Note that there is a substantialinfluence of the angle at which the unit operates, as indicated by thethree curves. When measuring inclined surfaces the angle has to beestimated and the values must be interpolated from the curve set.

In the present art the signal of interest is falsified by several errorfactors. Typically these can total up to 15, even 20% of the measuredvalue. The smaller the rebound energy, the greater the percentual errorcontribution. Especially for rebound values less than 20 the energyabsorbed by gravity and friction can be close to the rebound energy(article Dr. K. Gaede, volume 154 Schriften des Deutschen Ausschussesfür Stahl-beton).

To keep the influence of friction to a minimum, the apparatus must becarefully adjusted, cleaned and inspected frequently—all factorsincreasing the cost of the device and leading to a limited acceptance ofthe rebound method.

With the advent of digital electronics and LCD displays many companieshave “digitized” their instruments. Instead of having to read theposition of the mechanical drag pointer, these units feature a numericdisplay. Up to this point such instruments have simply converted thefinal position of the drag pointer into an electrical value either bycontacting means or non contacting (optical, Hall sensors etc.). Theindicator electronics can either be a separate box or mounted right onthe instrument. Such units have been in the market for over a decade.

FIG. 1 is a sectional view of a typical Schmidt hammer equipped with alinear potentiometer 1 to convert the position of the drag pointer 2 toan electrical value, which is transmitted to an external indicator unitvia a connector. All the other mechanical parts are 100% identical tothe original, mechanical Schmidt hammer. We note the mallet 3 thattravels on the guide rod 4 and hits the plunger 5 drawn by the impactspring 6. Housing 7 and release/reload mechanism 8 are mentioned for thesake of completeness.

“Integrated” models with numeric readouts are based on standardmechanical units equipped with sensing circuitry for the drag pointer.

All these solutions suffer from the problems inherent to mechanical dragpointer indicators:

1) The rebound value is dependent on the inclination of the surfaceunder test (effect of gravity on the mallet).

2) The readings remain dependent on the internal friction of theapparatus (mallet traveling on guide rod plus friction of drag pointer).

3) The transfer efficiency of the kinetic energy between the mallet andthe plunger is not constant due to the mismatch of the two masses.

4) The impact energy (length of spring) and the zero position of eachinstrument have to be manually adjusted, which increases cost and thechance of maladjustments.

5) The impact energy is dependent on the angle of incidence due togravity.

6) The readings remain dependent on the way the operator actuates theapparatus—vigorously or hesitantly (velocity of housing with respect tofixed coordinate system).

U.S. Pat. No. 5,176,026 (Leeb, Brunnner) (FIG. 2) describes an apparatuswhich measures the rebound travel of the mallet by means of a transducerconsisting in a reflective optical detector 7 a and a mallet 3 featuringgrooves 8 filled with an opaque substance on its entire length. Thisapproach eliminates the drag pointer and its friction, whereas the othererror sources (effect of gravity on mallet, friction of mallet on guiderod, zero position of spring) are still affecting the result.Furthermore the reflective sensor scheme is lacking due to itssusceptibility to dirt and fingerprints. This type of device has been oflimited commercial success. The implementation shown in FIG. 1, although“cruder” in its design, remains the state-of-the-art.

Attempts have been made to apply a technique that is used in assessingthe hardness of metals (U.S. Pat. No. 4,034,603) to the Schmidthammer—so far these efforts have failed.

In this technique—intended for shop use—the mallet directly impinges onthe sample and the housing of the impact device rests on the surfaceunder test.

Note:

a) The Schmidt hammer is used mainly under outdoor conditions and mustbe sealed against dust and moisture, therefore the mallet cannot impingedirectly on the surface under test, but must transfer its energy via theplunger 5. This design allows for a seal 9 between the moveable plunger5 and the instrument housing 7.

b) The loading and trigger mechanism of the Schmidt hammer is such thatthe unit rests on the plunger and not on the housing of the instrument.

DISCLOSURE OF THE INVENTION

Hence, it is a general object of the invention to provide an apparatusof the type above that measures a more accurate parameter indicative ofthe compressive strength of the solid under test.

This object is achieved by the apparatus and method of the independentclaims.

Accordingly, the apparatus is equipped with a sensor that measures atleast two rebound velocities of the mallet at different times during itsrebound motion. Using the at least two measured velocities, the controlunit of the apparatus then calculates a parameter that is indicative ofthe compressive strength of the solid.

This technique is based on the understanding that a measurement of thevelocity immediately after impact is more accurate than the conventionalmeasurement of the rebound height because the rebound velocity can bemeasured at an earlier stage of the rebound motion and therefore is muchless prone to errors due to gravity and friction. In addition, it isbased on the understanding that a single measurement of the reboundvelocity will suffer from errors due to the strong mechanicaldisturbances the instrument is subjected to during rebound. Thesedisturbances are primarily due to the repercussions of the impactsbetween mallet and plunger and between plunger and solid. However, sincethe mechanical disturbances only cause temporary glitches in themeasurement, the detection of two or more velocities allows to recognizeand/or eliminate their influence efficiently.

Advantageously, the apparatus is further adapted to measure at leastone, in particular at least two, inbound velocities of the mallet priorits impact against the plunger. The inbound velocity or velocities canbe used to further refine the measured parameter because knowledge ofthe inbound velocity allows to account for errors prior to the impact,such as gravity, friction and spring fatigue.

Advantageously, the apparatus is further adapted to compute the quotientof rebound versus inbound velocity, each taken at the same relativelocation between mallet and housing.

Advantageously, the plunger has a mass that is substantially equal tothe mass of the mallet. The advantages of this measure are two-fold. Onthe one hand, an equality of these masses ensures a full energy transferbetween mallet and plunger and therefore increases the accuracy of theinstrument. In particular, the impact brings the mallet substantially toa stop, while its full kinetic energy is transferred to the plunger.While the mallet is substantially stationary, the plunger hits the solidand bounces back to transfer all its remaining energy back to theplunger. In contrast to this, in prior art instruments the mallet isgenerally much heavier than the plunger. Therefore, the energytransferred to the plunger is smaller, and the mallet does not come to astop after impact and tends to hit the plunger several times, therebymaking the measurement unpredictable. In addition, only part of theenergy of the plunger is transferred back to the mallet, therebydecreasing the sensitivity of the device. Finally, using a mallet thatis lighter than prior art instruments allows to accelerate it to ahigher speed, thereby decreasing the time span for gravity to affect itsmotion and further increasing the accuracy of the instrument. Thereduced variation of impact velocity/energy helps to ensure the impactenergy stays within the tolerance specified by national standards. Also,the instrument has smaller overall mass.

The invention is particularly suited for the measurement of thecompressive strength concrete.

The invention relates to an apparatus as well as a method. Inparticular, it is noted that any method-related features of the claimsrelating to the apparatus can also be formulated as claims relating tothe method, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those setforth above will become apparent when consideration is given to thefollowing detailed description thereof. Such description makes referenceto the annexed drawings, wherein:

FIG. 1 shows a first prior art instrument equipped with a linearpotentiometer,

FIG. 2 is a second prior art instrument equipped with a mallet having areflective pattern on its entire length,

FIG. 3 is a graph showing the conversion curves for a prior art Schmidthammer,

FIG. 4 is a perspective view of key components of a preferred embodimentof the apparatus,

FIG. 5 shows the circuitry for processing the signals from the opticaldetectors,

FIG. 6 is an exploded view of the device,

FIG. 7 is a sample waveform as detected by the sensor, and

FIG. 8 is a graph showing conversion of “Q” factor to compressivestrength.

MODES FOR CARRYING OUT THE INVENTION

FIG. 4 shows the relevant parts of an advantageous embodiment of thedevice at the time of impact. The housing has been omitted forsimplicity (FIG. 6 shows the complete hammer unit). We note the mallet3, which travels along an axis A on a guide rod 4 and impinges on theplunger 5 drawn by the impact spring 6, with impact spring 6 acting as adrive mechanism for driving mallet 3 against plunger 5. These parts arevirtually identical to the classic implementation as e.g. described inU.S. Pat. No. 5,176,026. However, the mass of the mallet 3 is nowreduced to match the mass of the plunger. This is to ensure that theenergy of the mallet is transferred to the plunger in one single blow,when it travels toward the plunger and from the plunger to the malletupon return. In the classic unit there are multiple contacts of a randomnumber, whereby a part of the energy is lost. Further advantages of thisdesign have been described above.

Note that the impact spring 6 is the same as in prior art instruments;it features a spring constant of 0.79 N/mm and is extended to 75 mmyielding an impact energy of: E=2.22 Nm.

The mallet 3 weighs only 115 grams, therefore it travels with a highervelocity. V=sqrt (2*E/m)=6.21 m/s. The time of travel is thereforereduced and likewise the effect that gravity has on accelerating ordecelerating the mallet.

To measure the velocity of the mallet 3, it is equipped with a number ofcircumferential ribs 11 of trapezoidal cut that form projections on itscylindrical outer surface. These ribs are located such that upon passagethey interrupt the beam of a dual light barrier 12. The beam is orientedtangentially with respect to the mallet 3.

The dual light barrier 12 comprises an infrared light source 13 and adual light detector 14 having two sensor areas of 1 mm² and arrangedsuch that each of them will alternately be illuminated or shadowed uponpassage of the mallet 3 and its ribs 11 at the location of light barrier12.

As shown in FIG. 5, the output of the two sensor areas are connected toa differential amplifier 16 (TI INA321). Its output is connected to theA/D port of a control unit 24 (TI MSP430G) arranged on a printed circuitboard 17 within the device.

Note how the stationary part of the encoder, namely light barrier 12, isalso directly located on the circuit board 17. Circuit board 17 furthercarries all other electronic components, in particular a dot matrixdisplay 18 with its controller 23, a battery 22 and a single button 19to control the unit.

A two axis accelerometer 20 is also arranged on circuit board 17. Itserves multiple tasks. It monitors the acceleration of the housingduring the measurement phase and allows for compensation of itsmovement. It also detects the angle of inclination and can make fineadjustments to the measured parameters.

It is further used in lieu of a cursor key to scroll (move), swap ororient the text and symbols displayed on display 18. In particular, itcan detect which side of the device is up and which is down, and, basedon this, rotate the text in the display for best readability. It alsocan cause a long text or image displayed on the text to be scrolled ormoved along the display when the user tilts the display in a defineddirection.

A USB connector 21 is provided to charge the battery and to interfacethe unit to an external device, e.g. a PC.

FIG. 6 shows how the present design allows to encapsulate the entireunit in a housing 7 that completely seals it against dust and moisture.

FIG. 7 shows a typical signal such a sampled by the 12 bit A/D converterof control unit 24, where the approximate instant of impact betweenmallet and plunger is denoted by a line X.

As can be seen, a series of signal oscillations is detected prior toimpact and during rebound of the mallet. The frequency or period ofthese oscillations is indicative of the velocity of the mallet.

In the present embodiment, the absolute velocity is detected at leasttwice prior to impact and at least twice during rebound. The velocitiesare measured by computing the zero crossings of the signal and bycalculating the distances between consecutive rising or falling zerocrossings.

In the example of FIG. 7, at least a first and a second average inboundvelocity at different times are calculated prior to impact from theperiod I_(N1) between the last two falling zero crossings and from theperiod I_(P1) between the last two rising zero crossings. Similarly, afirst and a second average rebound velocity at different times arecalculated during rebound from the period R_(P1) between the first tworising zero crossings and from the period R_(N1) between the first twofalling zero crossings.

The sample rate of the signal from amplifier 16, indicated by the dotsin FIG. 7, is comparatively low with respect to the waveform to beanalyzed. In order to attain enough resolution, the precise occurrenceof the zero crossings is obtained by a linear interpolation between thetwo samples on either side of the zero line Z.

In essence the waveform detected by the photo interrupter issymmetrical. The right side is stretched by the factor ofinbound/rebound velocity.

Careful observation shows, however, a decrease of velocity as one movesfurther away from the point of impact X as a result of friction, gravityand spring back-force.

Therefore, advantageously, we determine the quotient of rebound toinbound velocity, the “Q” factor, by pair wise division of thecorresponding periods in close proximity of the point impact. The pairwise division is best carried out by dividing the rebound and impactvelocities that were measured on the same section of the mallet. Namely,the first rebound velocity v_(r1) derived from period R_(P1) is dividedby the first inbound velocity v_(i1) derived from period I_(N1) tocalculate a first Q-factor Q₁=100*(I_(N1)/R_(P1)) because both v_(r1)and v_(i1) were measured on the same rib 11 of mallet 3. Therefore,errors due to dirt or mechanical imperfections of the rib will beeliminated. Similarly, the second rebound velocity v_(r2) derived fromperiod R_(N1) is divided by the second inbound velocity v_(i2) derivedfrom period I_(P1) to calculate a second Q-factorQ₂=100*(I_(P1)/R_(N1)). The final Q-factor, which will be used tocompute the compressive strength of the solid, is calculated as theaverage Q=(Q₁+Q₂)/2.

More quotients could be averaged or processed using statistics, but onehas to consider that, due to friction, gravity and the spring force, thevelocity will change as the samples are taken further away from the timeof impact.

On the other hand the decrease of rebound velocity can be assessed andused to alert the user to the presence of excessive friction in thesystem.

The “Q” factor can be considered as the “true” rebound coefficient,since it is virtually unaffected by the aforementioned side effects. Theinventive method therefore does not set a new standard for assessingconcrete compressive strength, but will yield more reliable results overan extended range.

The inbound velocity can further be used to verify the impact energyspecified by international standards to be 2.207 Nm±6%.

Typically: E=0.5*0.115 kg*(6.21 m/s)²=2.22 Nm

Since the mass of the mallet is constant, monitoring the impact energysubstantially corresponds to comparing the inbound velocity to anallowable range of inbound velocities (or equivalently, comparing one ofthe periods of the inbound signal trace of FIG. 7, such as I_(N1) orI_(P1), to an allowable time range). The device can issue an alert ifthe inbound velocity does not fall within the allowable range. Byreading the accelerometer output the device can differentiate which partof the inbound velocity is caused by gravity and which by the drivemechanism.

(At this point the reader can see an important difference between theprior art U.S. Pat. No. 5,176,026 (Leeb, Brunner) where a reflectiveoptical scheme is used, however to assess the rebound travel of themallet and not its velocity, let alone the quotient of rebound toinbound velocity).

The Q-factor is a parameter indicative of the compressive strength ofthe solid. It can be converted to physical units, like N/mm², bycarrying out a large number of laboratory and field tests, where cubesmeasuring 150×150×150 mm are assessed with the inventive device andsubsequently crushed by means of a press.

A graph showing the relationship between compressive strength and the“Q” factor is given in FIG. 8. Note that there is but one conversioncurve for all possible angular orientations of the surface under test.For the reasons outlined above the curve is not unlike the oneapplicable to the mechanical Schmidt hammer. (As a coarse approximationone may set: Q≈R×1.12). The “Q” scale accommodates a wide range ofcompressive strengths, from 5 to 150 N/mm2 and beyond.

Calibration data describing the curve of FIG. 8 can be stored in thecontrol unit of the present device, thereby allowing it to convert themeasured “Q” factor to the compressive strength of a cube. Theconversion to compressive strength is done after having computed anaverage of a sufficient number of “Q” factors.

The result of the measurement can be displayed in the form of a barand/or as a number on display 18. The result can e.g. be the “Q” factor,the compressive strength of a cube as obtained by the curve of FIG. 8,or any other suitable value descriptive of the strength of the measuredmaterial.

As can be seen from FIG. 4 and as it also follows from the signals inFIG. 7, the projections or ribs 11 are arranged such that they arelocated in the range of the light barrier 12 at the time of impact,which allows to measure the inbound and rebound velocities immediatelyprior to impact and right at the beginning of the rebound motion. Hence,the measured velocities represent the situation during impact in anoptimum manner and are not falsified by friction, gravity or the brakingaction of spring 6 on mallet 3.

As mentioned, an accelerometer 20 is provided in the present device. Itsoutput signals are used by the control unit to correct the measuredparameter.

Advantageously, accelerometer 20 should at least be able to measure theacceleration along axis A, which allows to assess the influence ofgravity and sudden motions of the device's housing on the movement ofthe mallet and to correct the measured results.

In particular, if accelerometer 20 measures a constant, non-zeroacceleration along axis A during the measurements of the mallet motion(i.e. during the time span between release of the mallet and periodI_(P1) and R_(N1) in the example above), it is assumed that the housinghas not been accelerated during the measurements but that a constantgravity component was influencing the motion of mallet 3. In that case,the inbound velocities v_(i1) and v_(i2) as well as the reboundvelocities v_(r1) and v_(r2) need not be corrected.

If a non-constant, significant acceleration is measured along axis A,the same can be split up into a constant contribution representinggravity and a non-constant contribution representing an acceleration aof the housing. The constant acceleration is attributed to gravity,while the non-constant acceleration component can be integrated tocalculate the velocity of the housing at the times of the measurementsof the inbound and rebound velocities and added/subtracted from the sameaccordingly:

v_(rk)^(′) = v_(rk) + ∫₀^(t_(rk))a𝕕t  andv_(ik)^(′) = v_(ik) − ∫₀^(t_(ik))a𝕕twith k=1 . . . N and N being the number of velocity measurements priorand after impact (N=2 for the example above), v′ denoting the correctedvelocity measurements, t_(rk) the time span between the measurement ofv_(rk) and the release of the mallet, t_(ik) the time span between themeasurement of v_(ik) and release of the mallet, and a being themeasured acceleration along axis A.

The time of impact as well as the energy of the mallet during impactdepends on where the plunger is located at that time. In particular, itmust be noted that a small spring (denoted by reference number 10 in theprior art embodiment of FIG. 1) is arranged between guide rod 4 andplunger 5 to uncouple the plunger 5 from the guide rod 4 during impact.Depending on how strongly the user presses the instrument against thesolid under test, the length of that spring varies and therefore (sinceguide rod 4 is fixedly connected to housing 7) the impact position ofplunger 5 may vary. For example, if the user presses the device stronglyagainst the solid, the plunger will rest more deeply within theinstrument and the path length for accelerating mallet 3 will be shorteras compared to a situation where the user presses the device weaklyagainst the solid.

The position of the plunger during impact can be derived from theposition of the first and last oscillations of the signal of FIG. 7before and after impact X. In the example of FIG. 7, the time intervalΔt between the last falling zero crossing before impact X and the firstrising zero crossing after impact depends on the position of plunger 5during impact. This time interval Δt corresponds to the time spanbetween the time when the last edge of a rib 11 passes the sensor 13prior to impact and the time when the same edge passes the sensor 13during rebound. Instead of a rib edge, any other detectable mark on themallet could be used for this measurement.

In an advantageous embodiment, reference measurements are carried outwith the instrument against a sample of known compressive strength fordifferent plunger positions. For each measurement, the time interval Δtand the deviation of the measured parameter from a correct value of theparameter is recorded. This allows to establish heuristic calibrationdata describing how to correct the value of the measured parameter as afunction of time period Δt. This calibration data is stored in controlunit 24.

The described device has a large number of advantages. It particular, itaccurately records a parameter indicative of compressive strengthwithout the need for correction of the angular inclination of theapparatus. It accurately measures the velocity of the mallet immediatelybefore and after the impact in a non-contacting way, thereby yieldingresults that are substantially unaffected by gravity and friction aswell as by the braking action of spring 6 during rebound. It yieldsreliable results for a range of compressive strengths both above andbelow the currently attainable values, e.g. 5 to 150 N/mm². It providesa display featuring a user-friendly readout right on the unit. Further,the device is easy to assemble, to calibrate and to service.

While there are shown and described presently preferred embodiments ofthe invention, it is to be distinctly understood that the invention isnot limited thereto but may be otherwise variously embodied andpracticed within the scope of the following claims.

1. An apparatus for the non-destructive measurement of the compressivestrength of a solid, in particular of concrete, comprising a plungerdisplaceable along an axis and having a front end for probing the solid,a mallet displaceable along said axis to impact against said plunger, adrive mechanism for driving said mallet against said plunger, therebycausing said plunger to impact against said solid and to generate arebound of said mallet, a sensor measuring at least two reboundvelocities of said mallet at different times during said rebound, and acontrol unit calculating a parameter indicative of the compressivestrength of said solid depending said at least two rebound velocitiescomprising a plurality of projections on a surface of said mallet andwherein said sensor comprises a light barrier generating at least onelight beam interrupted by said projections.
 2. The apparatus of claim 1wherein said sensor is further adapted to measure at least one inboundvelocity of said mallet prior to impact against said plunger, andwherein said control unit is adapted to calculate said parameterdepending on said at least one inbound velocity.
 3. The apparatus ofclaim 2 wherein said control unit is further adapted to compare saidinbound velocity to an allowable range of inbound velocities and togenerate an alert if said inbound velocity does not fall within saidallowable range of inbound velocities.
 4. The apparatus of claim 3wherein said control unit is adapted to generate a quotient (Q) betweensaid inbound velocity and at least one of said rebound velocities. 5.The apparatus of claim 1 wherein said sensor is further adapted tomeasure at least two inbound velocities of said mallet at differenttimes prior to impact, and wherein said control unit is adapted tocalculate said parameter depending on said at least two inboundvelocities.
 6. The apparatus of claim 5 wherein said sensor is adaptedto measure at least a first inbound velocity and a first reboundvelocity when a first section of said plunger passes said sensor and asecond inbound velocity and a second rebound velocity when a secondsection of said plunger passes said sensor, and wherein said controlunit is adapted to calculate at least a first ratio (Q₁) of said firstrebound and impact velocities and a second ratio (Q2) of said secondrebound and impact velocities.
 7. The apparatus of claim 6 wherein saidcontrol unit is adapted to calculate an average of said ratios (Q1, Q2).8. The apparatus of claim 1 wherein said plunger has a masssubstantially equal to the mass of said mallet.
 9. The apparatus ofclaim 1 further comprising at least one accelerometer, wherein saidcontrol unit is adapted to correct said parameter depending on a signalgenerated by said accelerometer.
 10. The apparatus of claim 9 whereinsaid accelerometer is adapted to measure an acceleration along saidaxis.
 11. The apparatus of claim 9 further comprising a display fordisplaying at least said parameter, wherein said apparatus is adapted toorient and/or move a displayed text depending on an orientation of saidapparatus as measured by said accelerometer.
 12. The apparatus of claim1 wherein said control unit is adapted to measure an impact position ofsaid mallet against said plunger and to use said impact position whencalculating said parameter.
 13. The apparatus of claim 12 wherein saidsensor is adapted to detect a passage of a first mark on said plunger,wherein said control unit is adapted to measure said impact positionfrom a time interval (ΔT) passing between the passage of the first markprior to impact and during rebound.
 14. The apparatus of claim 1 whereinsaid projections are located to be positioned in a range of said lightbarrier at a time of said impact.
 15. The apparatus of claim 1 whereinsaid sensor comprises at least two light detectors subsequently shadowedby said projections during a passage of said mallet and an amplifiercircuit measuring a difference of signals from said two light detectors.16. The apparatus of claim 1 wherein said projections extend around acylindrical outer surface of said mallet.